该研究通过高压高温拉曼光谱和同步辐射红外反射光谱，系统研究了双层镍氧化物La₃Ni₂O₇在压力与温度下的结构相变与电子性质演变。拉曼测量证实了压力驱动的结构相变（从倾斜的Amam相到无倾斜的Fmmm或I4/mmm相），并观察到Fano线型出现，表明电子-声子耦合增强。高温数据揭示了该相变在常压下存在544 K的上限温度，完善了温压相图。红外反射率测量显示，相变伴随载流子密度提升近两个数量级，标志着从不良金属到良金属的交叉。实验建立了结构相变与电子性质强耦合的统一图像，超导性在约6-7 GPa处与无倾斜相的出现紧密相关，但高对称性与金属性本身不足以诱发超导，可能还需考虑密度波涨落等强电子关联效应。
The recent discovery of superconductivity in La3Ni2O7–δ with a transition temperature Tc close to 80 K at high pressures has attracted significant attention, due particularly to a possible density wave (DW) transition occurring near the superconducting dome. Identifying the type of DW order is crucial for understanding the origin of superconductivity in this system. However, owing to the presence of La4Ni3O10 and other intergrowth phases in La3Ni2O7–δ samples, extracting the intrinsic information from the La3Ni2O7 phase is challenging. In this study, we employed 139La nuclear quadrupole resonance (NQR) measurements to eliminate the influence of other structural phases in the sample and obtain microscopic insights into the DW transition in La3Ni2O7–δ. Below the DW transition temperature TDW ∼ 153 K, we observe a distinct splitting in the ±5/2 ↔ ±7/2 transition of the NQR resonance peak at the La(2) site, while only a line broadening is seen in the ±3/2 ↔ ±5/2 transition peak. Through further analysis of the spectra, we show that the line splitting is due to a unidirectional charge modulation. A magnetic line broadening is also observed below TDW, accompanied by a large enhancement of the spin-lattice relaxation rate, indicating the formation of magnetically ordered moments in the DW state. Our results suggest a simultaneous formation of charge- and spin-density wave orders in La3Ni2O7–δ, thereby offering critical insights into the electronic correlations in Ni-based superconductors.
该研究结合第一性原理计算、压力依赖的双轨道模型以及自洽FLEX和线性化Eliashberg方程，揭示了无限层La₀.₈Sr₀.₂NiO₂中压力增强且稳健超导的微观机制。结果表明，压力升高增大了动能尺度、降低了有效关联强度U/t、增强了层间杂化，并使空穴从La/Sr电荷库转移到关联的Ni区域；在低压下，动能尺度增大和体系趋近最佳中等耦合促进配对，而高压下压力诱导的自掺杂使体系进入过掺杂区，从而抑制超导，形成宽超导穹顶。尽管压缩使费米面显著三维化，与配对相关的自旋磁化率对q_z依赖较弱，仍主要在(π,π)附近出现峰值，因此Ni d_{x²-y²}主导的d波配对态在计算压力范围内保持稳定。这一受限的低能配对框架解释了该材料在兆巴级压缩下超导的异常稳健性。
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We report a 139La-NMR study of polycrystalline samples of multi(n)-layered nickelates, La3Ni2O7−δ (n = 2) and La4Ni3O10−δ (n = 3), at ambient pressure. Measurements of the nuclear magnetic resonance (NMR) spectra and nuclear spin relaxation rate (1/T1) indicate the emergence of a density wave order with a gap below T* ∼ 150 K for La3Ni2O7−δ and ∼130 K for La4Ni3O10−δ. The finite value of 1/T1 below T* indicates metallic ground states with the remaining density of states at the Fermi level (EF) under the density wave order. These features are attributed to multiple d electron bands with different characteristics. Above T*, the gradual decrease in 1/T1T upon cooling implies the presence of a band with flat dispersion near EF. From our microscopic probes, we point out that these nickelates (n = 2 and 3) possess similar electronic states despite the difference in the formal valence of the Ni d electron states, which provides a basis for understanding the novel high-Tc superconductivity under high pressures.
该研究通过结合线性太赫兹时域光谱与三阶谐波产生技术，系统探测了压缩应变(La,Pr)₃Ni₂O₇薄膜的超导配对对称性及正常态赝能隙。线性太赫兹光谱观测到超导转变温度以下低频谱权重的显著抑制，伴随弱相干峰以及趋近零温时仍存在的较大剩余电导，与无序s±波配对图像一致。非线性三阶谐波信号在进入超导态后急剧增强，但其响应在超导转变温度以上持续存在，并在约100K处出现扭折，结合角分辨光电子能谱在同类薄膜中观测到的类似温度尺度，将其归因于正常态赝能隙。研究确立了(La,Pr)₃Ni₂O₇为具有s±波类配对的体超导体，且该超导态与另一有序态共存并可能相互竞争，为探索超越铜基和铁基的非常规超导机制提供了新平台。
本研究利用偏振分辨拉曼散射并结合两轨道模型计算，揭示了三层镍酸盐La4Ni3O10密度波的多轨道特性。在声子部分，氧键伸缩模式在密度波转变温度T_DW附近出现明显的强度异常，而声子线形基本对称且频移和线宽变化微小，表明电子-声子耦合温和，密度波主要由电子不稳定性驱动而非晶格驱动。电子拉曼散射中，随着温度降低，连续谱权重在约114 meV（约910 cm⁻¹）以下显著耗尽，并在该能量处出现一个偏振相关的尖锐峰，该峰在B₂g通道中最强，B₁g通道中几乎缺失。通过构建同时包含Ni-3dₓ²₋ᵧ²和Ni-3d₂²轨道并在轨道空间中保留拉曼顶点的两轨道密度波态模型，计算确认该峰对应密度波能隙2Δ_DW，其打开表现为非相干和非平均场行为，且光谱响应必须依靠两个轨道的混合贡献，无法还原为任一单轨道投影。这些结果表明La4Ni3O10的密度波不稳定性具有本征的多轨道起源，为理解其与超导的竞争或共存关系提供了关键实验与理论依据。
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